The present invention relates generally to a filter for differentiating true control signals from false control signals that result from the coupling of extended parallel control and power lines. More particularly, the invention relates to a switching filter circuit producing a low impedance control input on a normally high impedance input line to discriminate false control signals induced on the control input line by the capacitive, inductive, or resistive coupling of the control line with a power line.
Control circuits, such as motor contactor controls, often are controlled remotely by long control lines which may extend from 100 feet to several miles and are typically run in parallel with other power lines, including the power line that supplies power to the motor and the control circuit. It has been found that running these lines in parallel over extended distances can cause false control signals on the control line due to capacitive, inductive, or resistive coupling between the lines. For example, where a power line and a control line are run in parallel over an extended distance, the inherent capacitive coupling between the lines can act as a near short circuit during the initial power up on the power line. This short circuit can produce a signal voltage on the control line to a control logic input of the control circuit which can then cause contactor closure where no actual control signal is intentionally provided. This can cause a motor to start momentarily when unexpected. Many such control circuits have relatively high input impedance which can compound the problem of capacitive, inductive, or resistive coupling. High input impedance in a control causes the line capacitance to discharge slowly.
Devices which may be remotely controlled by a control line in close proximity to other power lines, include programmable logic controllers, or PLCs, which are available with various types of input and output stages operable at AC or DC voltages. The outputs typically couple or decouple power to various operating elements which can include contactors or starters for motors, actuators for hydraulic or pneumatic valves, and various other devices that are either powered from the PLC outputs or include switching means that are triggered by the PLC outputs. Such PLCs are programmed to activate, or trigger, the output of the PLC in response to conditions at the PLC inputs. Many such controllers trigger on AC logic inputs and to differentiate noise and some false signals, include internal logic circuits that require several consistent, successive, and coincident charges on the logic inputs to activate. These controllers also will only detect such signals that are positive and occur after a zero-crossing. Although such safeguards function adequately in many applications, these controllers are being used in applications where the control lines are run in parallel with power lines at greater and greater distance. These distances inevitably cause greater capacitive and inductive coupling, thereby causing false control signals that the controller was never designed to compensate for.
Where capacitive coupling can cause a false signal on initial power up, inductive coupling can create false signals when the power line is opened. Inductive coupling can continue to supply power on the control line thereby keeping the motor contactors closed. Resistive coupling can occur with poorly insulated or wet wires thereby creating false signals at random times.
One prior art attempt at dissipating or compensating for such false signals includes adding a resistor across the control line to ground. However, the longer the control line, the greater the coupling between the lines which can create high powered false signals. To dissipate these signals, resistors with lower resistivity are required. However, the smaller the resistor, the more heat that is continuously produced when a true control signal is maintained on the control line input. If a larger resistor is used to minimize heat dissipation, the line capacitance is discharged too slowly. In other words, adding a resistor that is large enough to avoid excessive heat buildup, results in ineffective shunting of the false signals created by the leakage capacitance, inductance, and/or line resistance.
It would therefore be desirable to provide an add-on switching filter that could be plugged directly into such a controller, requires no separate power supply, and is capable of shunting false signals across a relatively low resistance path during a strategic portion of the line cycle to avoid excess power waste, thereby dissipating or compensating for line coupling.